Consumer electronic products such as televisions, digital cameras, cellular telephones, media content players, tablet PCs, etc., are designed, manufactured and marketed for the purpose of satisfying the substantial consumer demand for electronic equipment that is intended for everyday use. Data storage components play an important role in the operation of such devices. Data storage components can include RAM, PSRAM, ROM, flash memory, etc.
Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. Flash memory is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. Flash memory is a specific type of Electrically Erasable Programmable Read-Only Memory (EEPROM) that is erased and programmed in large blocks. Example applications include data storage for personal digital assistants (PDAs), laptop computers, digital audio players, digital cameras and mobile phones. Other applications include game consoles, where flash memory can be used instead of other types of EEPROMs or battery-powered SRAM for game save data.
PSRAM is dynamic RAM with built-in refresh and address-control circuitry to make it behave similarly to static RAM (SRAM). It combines the high density of DRAM with the ease of use of true SRAM. PSRAM is used in the Apple iPhone™ and other embedded systems.
In electronic applications where the memory subsystem includes both volatile (e.g., RAM) and non-volatile (e.g., flash) memory components, the memory components can either share the same bus or use separate busses. Memories that share the same bus need to have the same pin-out and throughput performance, since it can be difficult for a processor to manage memory components of varying speeds on the same bus. An example of a bus sharing memory subsystem is a memory subsystem that includes parallel NOR flash (non-volatile) and PSRAM (volatile) memories. In memory subsystems that use separate buses, the processor can manage each of the memory components independently, without bus contentions. However, independent bus systems have to support separate traces for both volatile and non-volatile memory components. It should be appreciated that independent bus subsystems are optimized for high throughput performance and place less of a focus on cost (such memories include parallel NOR Flash and Dynamic RAM memories). In contrast, bus sharing systems save costs that are directly related to the reduced number of traces that they feature.
Conventional bus sharing subsystems that feature a parallel NOR flash/PSRAM infrastructure can require 40 or more traces in order to match the data, address, and control lines provided by a memory interface. Reducing the component cost of packages that contain parallel NOR flash and PSRAM components is achieved by using smaller packages and/or reducing the size of the die upon which these circuits are formed. However, reductions in the size of the package is limited by the pads required to service the die as a die cannot be reduced in size beyond that which is necessary to accommodate required pads. Likewise, the number of required traces limit achievable package size reductions. Accordingly, although conventional parallel NOR flash/PSRAM memories provide reduced traces as compared to independent bus systems, the prospect for significant additional reductions as will be required to meet the continuing demand for decreased package size and increased cost savings is limited.